Farmers lung is a hypersensitivity pneumonitis caused by the inhalation of Thermophilic actinomycetes spores and possibly other agents found in moldy forage dust. Experiments are under way to establish an animal model of this disease in guinea pigs in order to establish the nature of the patho-immune response of the lung. In particular, the relative roles played by an Arthus type reaction versus a delayed hypersensitivity response will be assessed. Included in these studies will be the measurement of the function of alveolar macrophages in the presence of the spores, localization of antigen-antibody complexes by immunofluorescence and measurement of macrophage migratory inhibition factor (MIF). Corollary studies include the fractionation and physiochemical characterization of the antigen extracts obtained from Micropolyspora faeni, Thermopolyspora vulgaris, and Thermopolyspora viridis in order to produce better material for skin-testing and serological testing for the detection of hypersensitivity to these organisms in man.